This invention relates to a belt type infinite variable-speed drive having a movable pulley portion that is moved axially by a ball-screw mechanism.
A belt type infinite variable-speed drive has a fixed pulley portion and a movable pulley portion around which is wound a transmission belt. These pulley portions are mounted on a rotary shaft with their belt-receiving surfaces opposing each other. By axially moving the movable pulley portion by virtue of a direct acting actuator, a winding diameter of the transmission belt is changed in a stepless manner. As direct acting actuators, there are hydraulic actuators and mechanical actuators such as ball-screw mechanisms.
In a ball-screw mechanism, a nut having helical thread grooves formed in its inner periphery is mounted on a threaded shaft having helical thread grooves formed in its outer periphery. Between opposed thread grooves of the threaded shaft and the nut, a plurality of balls are rollably received so that the threaded shaft and the nut can smoothly move axially relative to each other. The nut has ball circulating portions for infinitely circulating the balls between the opposed thread grooves. Many of such ball-screw mechanisms are of a type in which, by axially moving the nut with axial movement of the threaded shaft prevented so as to move the nut relative to the threaded shaft, a moving member coupled to the nut is linearly moved.
Since belt type infinite variable-speed drives using a ball-screw mechanism as a direct acting actuator allows for a more compact design than that possible when using a hydraulic actuator, such variable-speed drives are used as infinite variable-speed drives for automobiles in which mounting space is limited, particularly small cars.
A prior art belt type infinite variable-speed drive is described in JP patent publication 8-33170. In this belt type infinite variable-speed drive, by turning a nut with a threaded shaft of a ball-screw mechanism rotatably and axially movably supported to a support member provided integrally with a casing, a movable pulley portion as a movable member coupled to the nut is linearly moved in an axial direction.
Since a belt type infinite variable-speed drive using the above-described conventional ball-screw mechanism is of a type in which the nut is turned, circumferential positions of ball circulating portions provided on the nut change with rotation of the nut. Since there is a clearance between the ball circulating portions and balls to smoothly circulate the balls, the balls in the ball circulating portions cannot bear loads such as moment loads and radial loads that act on the ball-screw mechanism. Also, directions of loads that act on the ball-screw mechanism of the belt type infinite variable-speed drive are substantially constant.
In a conventional belt type infinite variable-speed drive in which the nut turns, portions where ball circulating portions are provided, which are low in resistance to load, are periodically oriented in a load direction. Thus, it is necessary to additionally increase rigidity of the ball screw mechanism, so that no rational design is possible. Thus, further compactness of the belt type infinite variable-speed drive is impossible.
Also, since there is a clearance between the ball circulating portions and the nut, when the ball circulating portions turn together with the nut, balls vibrate and collide against walls of the ball circulating portions. This produces noise and wears the balls and the walls of the ball circulating portions.
An object of this invention is to provide a belt type infinite variable-speed drive of compact and rational design using a ball-screw mechanism, and to lessen noise.